Coherent Detection With a GaAs/AlGaAs Multiple Quantum Well Structure

A GaAs/AlGaAs multiple quantum well (MQW) structure has been investigated as a coherent detector using a CO2-laser local oscillator (LO). At an operating temperature of 77 K, an LO power of 10 mW, an LO wavelength of 10.2 μm, and a bias voltage of 2.0 V, a 150-μm-diam detector displayed an external quantum efficiency η0 of 11% and a 3-dB electrical bandwidth BIF of 1.5 GHz. Under the same conditions the noise-equivalent power density (NEPHET/Δf) at an intermediate frequency of 1.5 GHz was 5.5×10-19 W/Hz, which is within 50% of the photon-noise limit for this detector. A 75-μm-diam detector with a lower-capacitance bond wire was found to have a BIF of 8 GHz and an NEPHET/Δf of 4.1×10-19 W/Hz with only 2.5 mW of LO power. The photoelectron lifetime in the MQW structure at 77 K was estimated to be 2.5 ps corresponding to an intrinsic detector bandwidth of 64 GHz.